Cargo containers are typically used to transport goods internationally and domestically. Large numbers of such containers are loaded and unloaded at ports on an ongoing basis. Due to the large number of containers, port inspectors may not be able to open the containers to inspect their contents. This can pose a security risk.
To address the security risk introduced by an inability to open and inspect the contents of cargo containers, cargo inspection devices have been developed that scan the interiors of the containers without requiring inspectors to open the containers. Some cargo inspection devices perform radioscopic examination of cargo containers using an X-ray beam or gamma beam that can penetrate the container to identify its contents, for example by transmitting the beam through the container. A detector receives X-rays that have penetrated the shipping container without being absorbed or scattered, and produces an image of the contents of the shipping container. The image can be displayed to an inspector who can perform visual inspection of the contents. Materials with higher effective atomic number (Z) have a higher density, thus resulting in greater attenuation of the beam. Portions of the image showing greater X-ray attenuation may alert the inspector to the presence of a high-Z material for which further inspection is appropriate, e.g., a radioactive material.
Some X-ray based cargo inspection devices generate X-rays using a linear accelerator configured to produce a single energy X-ray beam. However, one shortcoming of such a single energy system is that it may not be possible to uniquely determine whether the X-rays are attenuated by a material that has a particularly high Z, or whether the material simply is very thick, both of which may cause similar attenuation of the X-rays. Other cargo inspection devices use dual energy linear accelerators that are configured to emit two different energy level X-ray beams. With a dual energy X-ray inspection system, materials can be discriminated radiographically by alternately irradiating an object with X-ray beams of two different energies. Dual energy X-ray inspection systems can determine a material's mass absorption coefficient, and therefore the material's Z. Differentiation is achieved by comparing the attenuation ratio obtained from irradiating the container with low-energy X-rays to the attenuation ratio obtained from irradiating the container with high-energy X-rays. Discrimination is possible because different materials have different degrees of attenuation for high-energy X-rays and low-energy X-rays, and that allows identification of low-Z-number materials (such as but not limited to organic materials), medium-Z-number materials (such as but not limited to transition metals), and high-Z-number materials (such as but not limited to radioactive materials) in the container. Such systems therefore may provide an image of the cargo contents and identify the materials within the container.
The ability of dual energy X-ray inspection systems to detect the Z number of materials being scanned enables such inspection systems to automatically detect the different materials in a container, including radioactive materials and contraband such as but not limited to cocaine and marijuana. However, previously known dual energy X-ray inspection systems use a standing wave linear accelerator that is vulnerable to frequency and power jitter and temperature fluctuations, causing the beam energy from the linear accelerator to be unstable when operated to accelerate electrons to a low energy. The energy jitter and fluctuations may create image artifacts, which cause an improper Z number of a scanned material to be identified. This may cause false positives (in which a targeted material is identified even though no targeted material is present) and false negatives (in which a targeted material is not identified even though targeted material is present).
Moreover, although previously known X-ray based inspection systems may allow for some degree of material discrimination, particularly high-Z materials may require additional inspection to determine whether the materials are radioactive or otherwise dangerous. Such inspection may be performed manually by the inspector, which may be time consuming and dangerous. Alternatively, a cargo container with a particularly high-Z material may be interrogated using neutrons so as to enhance identification of the material. For example, such neutrons may generate gamma rays when they interact with a material. The gamma rays may be detected with an appropriate detector, the output of which may be analyzed to identify the materials based on the spectral characteristics of the gamma rays. Or, for example, such neutrons may generate detectable fission neutrons if they interact with a radioactive material. However, previously known neutron generators are typically relatively expensive, and so any given cargo inspection facility may have only a single such generator available, and the container may need to be physically be moved near the generator, which again may be time consuming and dangerous.
Some effort has been made to simplify the process by which cargo containers may be scanned both with X-rays and with neutrons. For example, U.S. Pat. No. 7,551,714 to Rothschild discloses a combined X-ray computerized tomography (CT) and neutron material identification system. The system includes an X-ray CT system, e.g., an X-ray tube, linear accelerator, or radioactive source, to scan a rotating cargo container with a horizontal fan beam of X-rays. An array of detectors on the other side of the container generates a three-dimensional image of the interior of the container, and any suspect regions are identified. Then, a separate neutron system, e.g., a sealed deuterium-tritium (DT) tube or high-intensity plasma neutron source, irradiates specified portions of the container with a horizontal collimated beam of high-energy neutrons. Rothschild discloses that gamma rays emitted from the suspect region are detected by energy-resolving detectors, and that the energy spectrum is analyzed and compared to a library of energy spectra of threat items found in a database. Rothschild discloses that the X-ray CT scan can be performed using dual-energy CT, resulting in a reduced number of suspect regions that will need to be interrogated by the neutron system. However, Rothschild's proposed system is cumbersome because it requires rotation of the cargo container and separately controlled X-ray and neutron systems.
U.S. Pat. No. 5,838,759 to Armistead discloses a system that uses a single beam for a combination of X-ray inspection and neutron-induced gamma-ray spectroscopy employing a “photoneutron probe.” The system uses a commercial linear accelerator X-ray source to scan a cargo container to obtain an X-ray image. If nothing unusual is detected, the object is cleared. However, if suspicious shapes, densities, or compartments are revealed, the X-ray source is temporarily converted into a neutron source by causing an actuator to insert a beryllium (Be) sheet, i.e., a beam converter, into the X-ray beam, and moving the container or the detector so as to irradiate the suspicious region with neutrons. Contraband, if present, would absorb neutrons and emit gamma rays and/or fission neutrons that may be used to characterize the scanned materials. Although the same X-ray source may be used to generate both X-rays and neutrons, making the system somewhat less cumbersome than the system proposed by Rothschild, the mechanical aspects of Armistad's proposed system will necessarily increase the amount of time required to fully analyze a cargo container.